1. Field of Invention
The invention relates to a fuel injection device.
2. Description of Related Art
A fuel injection device is used for supplying fuel to an internal combustion engine or the like, and is generally provided with a valve element that is movable in its axial direction within the body of the fuel injection device, and is operated to open and close injection holes (which will be referred to as “injection hole valve element.” The injection hole valve element has a distal end portion that is provided for opening and closing injection holes, and a base portion positioned at a side opposite from the distal end portion. During a closed valve state of the injection hole valve element, the base portion receives high fuel pressure from a control chamber.
An actuator used for opening and closing the injection hole valve element does not directly open or close the injection hole valve element, but opens and closes a control valve element that is disposed within the control chamber. When the control valve element is pushed to open by the actuator, fuel flows back from the control chamber to a fuel tank or the like, thus reducing the fuel pressure in the control chamber. In this case, the fuel pressure on the base portion of the injection hole valve element drops whereas high fuel pressure continues to act on the distal end portion of the injection hole valve element. This pressure difference moves the injection hole valve element in a valve opening direction to open the injection holes, overcoming the force of a valve closing spring that urges the injection hole valve element in a valve closing direction.
When the thrust of the actuator is removed and therefore the control valve element is moved to a closed position by the spring and the like, the fuel pressure in the control chamber rises due to high-pressure fuel flowing into the control chamber. Therefore, the pressure difference between the fuel pressure acting on the distal end portion of the injection hole valve element and the fuel pressure acting on the base portion thereof becomes small so that the injection hole valve element is moved in the closing direction by the valve closing spring, and therefore closes the injection holes.
If the actuator employed in the fuel injection device is an actuator whose amount of expansion (i.e., amount of displacement) is small, such as an electrostriction actuator or the like, it is difficult for the actuator to directly open the control valve element. In a typical construction, therefore, a displacement amplification chamber is formed between a small-diameter piston that contacts the control valve element and a large-diameter piston that contacts the actuator. The amount of expansion of the actuator is amplified by the displacement amplification chamber, and the amplified displacement is exerted on the control valve element to move the control valve element to an open position.
More specifically, when the large-diameter piston is displaced by expansion of the actuator to open the control valve element, the displacement amplification chamber displaces the small-diameter piston in such a manner that the capacity of the chamber is maintained; therefore, the small-diameter piston undergoes an increased displacement. Thus, it becomes possible to reliably move the control valve element to the open position.
Generally, the displacement amplification chamber is filled with low-pressure fuel. It is ideal that a fixed capacity of the chamber be always maintained. In reality, however, as the large-diameter piston is displaced to open the control valve element, the fuel pressure in the displacement amplification chamber rises so that fuel in the displacement amplification chamber leaks out into a low-pressure fuel passageway via small gaps around the large-diameter piston and the small-diameter piston. Thus, the capacity of the displacement amplification chamber gradually decreases.
If such a reduced capacity of the displacement amplification chamber is left undealt with, a positional deviation of the small-diameter piston results, so that the control valve element cannot be reliably moved to the open position although a necessary amount of displacement of the small-diameter piston can be provided. Therefore, after the large-diameter piston is returned by contracting the actuator in order to close the control valve element, it is necessary to replenish the displacement amplification chamber with the amount of fuel lost, from the low-pressure fuel passageway.
Japanese Patent Application Laid-open No. 2001-248523 proposes a special filter disposed at an inlet of a high-pressure fuel passageway of a fuel injection device. However, this filter is not able to completely remove small extraneous particles or the like from fuel. Therefore, such extraneous particles may enter the fuel injection devices together with fuel. If such extraneous particles enter the displacement amplification chamber when the displacement amplification chamber is replenished with fuel from the low-pressure fuel passageway, the extraneous particles will impede the smooth sliding of the large-diameter piston or the small-diameter piston, resulting in the unreliable opening and closing of the control valve element.